This invention relates to a method for constructing an in situ retort zone in an oil shale deposit or the like and more particularly relates to an in situ retorting method and system for recovering products from an oil shale deposit or the like.
Oil shale deposits are shale formations wherein useful hydrocarbons exist in the form of "kerogen". While kerogen, which is a solid or semisolid, is for all practical purposes immobile within the shale, it is well known that liquid and gaseous hydrocarbons can be recovered by heating the oil shale. In recovering hydrocarbons from oil shale by use of heat, two basic techniques have evolved: surface retorting and in situ retorting.
Surface retorting involves mining the oil shale, transporting it to the surface, crushing the shale, and then passing it through a surface retort to extract the recoverable hydrocarbon products. Although surface retorting has been relatively successful in recovering hydrocarbons, problems inherent in this process (e.g., cooling and disposal of spent shale) have seriously deterred any widespread commercial application of this process.
In an in situ process, on the other hand, the retort zone is formed directly within the oil shale deposit. In accordance with known procedures, this zone normally takes the form of several individual rooms within a defined gallery area, each room being filled with rubblized shale for retorting. The rooms are formed by first removing a portion (e.g., 5 to 40%) of the shale within the defined room area and then rubblizing the surrounding shale into the void areas by explosives or other mining techniques. The rubblized shale is then retorted by either in situ combustion or by passing externally heated gas through the shale, and the resulting products are recovered through appropriate passages to the surface. Although the cooling and disposal problems inherent in surface retorting are substantially reduced in an in situ retorting process, other problems arise that must be considered in making an in situ operation commercially feasible.
Specifically, the retort zone should be constructed or laid out so that the maximum amount of the oil shale lying within the zone is actually subjected to retorting. This presents a problem since, in forming rooms of rubblized shale by present mining methods, it is necessary to leave substantial amounts of shale untouched in order to form the walls which define and separate the retort rooms. Due to the relatively impermeable nature of oil shale, only a minute portion of these solid walls will be retorted when the rubblized shale within a respective room is retorted, and the hydrocarbons in most of these walls will not be recovered. Therefore, for maximum utilization of the natural resources within a retort zone, the room walls should be formed so as to contain the least practical volume of shale; hence, they should be as thin as safety and operating procedures will allow.
However, as the thickness of the room walls decreases, the likelihood of such thin walls cracking or leaking during a retorting operation increases. Since it is desirable, at least from a commerical standpoint, to commence retort operations as soon as a gallery of rooms is ready, any off-gas from a room being retorted which might leak through a too thin or cracked room wall would pose a severe hazard to any personnel working in or preparing an adjoining room or gallery.